Frozen Guava Juice Concentrate - Where can I buy it?

I'm trying to make a Hawaiian-style guava cake, and the recipe calls for frozen guava juice concentrate. I've searched high and low to no avail. I just checked out Evergreen Market on Mission at 21st, and though they had several varieties of guava paste and nectar, there was no frozen concentrate to be found.

Not to get off track but have you considered using the paste or nectar? If it has high guava content it might make for a less cloying version of the cake (also, you can adjust the sweetness by adding your own sugar).